A person does not need expensive machines or a sports club to stay healthy. Fast walking for about thirty minutes a day is enough to help the body. Walking makes the heart and the bones stronger. It helps to control weight. It also lowers stress and improves mood. It suits most ages and needs no special skill. Doctors advise a person who is not used to sport to start with short distances. Then he should increase them slowly, so that he does not get tired out or hurt. It is best to walk when the temperature is moderate, to wear comfortable shoes, and to drink water before and after the walk.

Question: How many minutes of fast walking a day are enough, according to the text?